Coating of various substrates with metal films has been employed for such applications as decoration, improvement of electric characteristics and optical functions. Such metal films are employed for making use of intrinsic physical properties of the metals such as luster, shade, electric properties and optical properties in accordance to the application, while it is desired that the physical properties remain stable for a long period of time.
In the case of metal films used in optical applications including reflector mirror such as those represented by silver and aluminum, such characteristics as reflectivity, spectral characteristic and durability have great importance. A reflector mirror for a liquid crystal image projector or a light tunnel (multi reflector) for DLP projector™ (manufactured by Balzers Limited), for example, has a metal film such as silver film formed as a specular surface on a glass substrate.
However, since the optical system of a liquid crystal image projector uses many reflector mirrors, it is required to minimize the loss of light caused on the reflecting surface. Also because unevenness in the spectrum of the reflected light has direct influence on the color consistency of the projected image, the reflection characteristic is required to be flat over the region of wavelengths to be used. Since the light rays are directed to the reflecting surface in a range of incident angles from 10 to 50°, variations in the reflection characteristic with the change in the incident angle must be minimized.
Meanwhile a metal film such as thin silver film formed by vacuum deposition is extremely vulnerable to the influence of the environment, and is subjected to white clouding caused by an oxide film formed on the surface thereof in about 24 hours when left in the atmosphere as it has been vapor-deposited, and the metal film may be blackened and peel off in the worst case. Also when the metal film does not sufficiently adhere to the base material, water may infiltrate into the interface with the base material and cause erosion.
Metal oxide films include SiO2 film and titanium oxide (TiO2, Ti2O3) film that are formed alternately as anti-reflection films on the surface of glass; TiO2 film used as a coating of hydrophilic (anatase) material film on the surface of an outdoor protective plate and optical component such as mirror and lens; Cr2O3 film used as a high insulation film for coating on the surface of a glass member (so-called multi-form glass) that is used for the glass insulator of cathode ray tube electron gun; and Cr2O3 film used as a release film for coating on the surface of a ceramic mold made of SiC ceramic material or the like for forming glass lenses. There are many other metal oxide films used in diverse applications.
Ion plating process is often employed as the method for forming thin films whereby a metal film or a metal oxide film is formed on the surface of a substrate such as glass. In the ion plating process, atoms evaporated from a heated evaporation source are partially ionized by glow discharge or plasma generated by a high frequency antenna under a reduced pressure, thereby to form a metal film or a metal oxide film by vapor deposition on a substrate having a negative bias voltage applied thereto. Metal films and metal oxide films are also formed by the vacuum deposition method employing resistive healing wherein an evaporation source is heated and evaporated by resistive heating and is deposited on the substrate surface in vacuum.
However, since the metal films obtained by these methods have rough surfaces with the crystal being poorly oriented, it has been difficult to achieve high reflectivity. There have also been such problems as the while clouding of the metal film surface due to oxidation and weak bonding with the substrate surface as described previously.
In the case of chromium oxide film used as an electrical insulation film, for example, it is known that the resistivity changes depending on the oxygen content included in the film. To counter this problem, various attempts have been made to obtain chromium oxide films having high resistivity by accelerating oxidation when forming the thin film. However, since sufficient reaction energy cannot be obtained simply by accelerating oxidation in the case of vacuum deposition process in which chromium oxide is evaporated and deposited in oxygen gas atmosphere, the film tends to become deficient of oxygen resulting in insufficient resistivity. In the case of ion plating and sputtering processes, oxidizing reaction is accelerated but the substrate is attacked by carrier gas or the like thus making it difficult to control the composition.
When a plastic material is used as the substrate, sufficient bonding of a film thereto cannot be achieved by the prior art method, and it has been difficult to form an insulation film of practical value. Moreover, since the substrate cannot be maintained at a desired low temperature, materials that can be used in the substrate have been limited.
The thin film forming methods of the prior art described above have such a problem that a metal oxide film having a high packing density cannot be obtained. The packing density of a metal oxide film formed by the ion plating process or the sputtering process of the prior art is from 0.8 to 0.95 at the most. Such a low packing density (in other words abundance of voids in the film) affects the optical characteristics of the film, and therefore the metal oxide film of the prior art has a low and unsatisfactory reflectivity in the region of visible light.